The present invention relates to a frequency interpolating device and a frequency interpolating method capable of improving spectrum distribution of a band-limited signal.
Distribution of data of the MPEG1 audio layer 3 (MP3) format and distribution of music and the like by frequency modulation (FM) broadcast, television sound multiplex broadcast and other methods are prevailing nowadays. These methods generally eliminate frequency components of about 15 kHz or higher of music and the like in order to avoid an increase in data amount and an expansion of an occupied band width to be caused by an excessively broad band.
Music and the like whose frequency components at a predetermined frequency or higher are removed have generally a poor sound quality. Signals substituting the removed frequency components are added to improve the sound quality, as disclosed in JP Laid-Open Gazette No. 7-93900.
According to the approach disclosed in JP Laid-Open Gazette No. 7-93900, a PCM digital audio signal is passed through a low pass filter and its output signal is multiplied by a signal containing absolute value components of the output signal to generate distortion.
An audio signal reproducing apparatus disclosed in JP Laid-Open Gazette No. 7-93900 generates harmonic waves by distorting the waveform of an output audio signal with a limitter circuit and the like. It is indefinite that such harmonic waves are approximate to those contained in the original audio signal.
The invention has been made to solve the above-described problem associated with prior art. A first object of the present invention is to provide a frequency interpolating device and a frequency interpolating method capable of recovering a signal approximate to an original signal from a predetermined conversion signal obtained from a band-limited signal of the original signal, particularly a frequency interpolating device and a frequency interpolating method capable of recovering an audio signal of a high sound quality.
According to the prior art, even if the frequency components at a predetermined frequency or higher are not necessary to be removed, an audio signal of music or the like is compressed into the MP3 format or the like so that the band of the audio signal is generally limited.
Even if original sounds or the like represented by a PCM digital audio signal have no frequency components higher than the pass band width of a low pass filter, a conventional device adds unnecessary high frequency components not contained in the original sounds or the like. The quality of an output audio signal is degraded more than it is passed through a low pass filter and additional signal processing is not performed.
Under such circumstances, a second object of the invention is to provide a frequency interpolating device and a frequency interpolating method capable of recovering a signal approximate to an original signal even from a signal mixed with a signal representative of the original signal whose spectrum components in some bands were removed and a signal representative of the original signal which has no spectrum components in these bands.
In order to achieve the first object of the invention, in a frequency interpolating device for receiving an input signal of an original signal whose frequency components in a particular frequency band was suppressed, approximately recovering suppressed frequency components and reproducing a signal approximate to the original signal, short period spectra are obtained from a frequency band having frequency components not suppressed, a short period spectrum in the suppressed frequency band are estimated by paying attention to repetition of a spectrum pattern at a predetermined frequency interval, and in accordance with this estimation, a signal containing the frequency components in the suppressed frequency band is synthesized and added to the input signal. More specifically, in the frequency interpolating device of the invention, the repetition of the spectrum pattern is judged from a correlation coefficient between a spectrum pattern in a first frequency band having a predetermined band width and frequency components not suppressed near the suppressed frequency band and a spectrum pattern in a second frequency band adjacent to the first frequency band having the predetermined band width.
If the spectrum pattern in the first frequency band and the spectrum pattern in the second frequency band have a high correlation coefficient, a replica of the spectrum pattern having a correlation is coupled to interpolate the frequency components in the suppressed frequency band.
With this frequency interpolating device, a portion of a spectrum of a signal to be interpolated having a high spectrum distribution correlation is added along an envelope line to the high frequency side of the signal to be interpolated to thereby expand the band. The added spectrum can be regarded as some harmonic components of the original spectrum. Therefore, if the signal to be interpolated has a limited band, the signal with the expanded band is approximate to the original signal before the band limitation. If the signal to be interpolated is an audio signal, the audio signal of a high sound quality can be recovered from the signal with the expanded band.
In the frequency interpolating device of the invention, the intensities of the frequency components to be synthesized are determined from a spectrum envelope of the suppressed frequency band estimated from a spectrum envelope of the frequency band not suppressed. Preferably, the particular frequency band is a high frequency band, and an upper limit frequency of the first or second frequency band is a lower limit frequency of a suppressed high frequency band.
If the interpolation band contains the highest frequency spectrum of the signal to be interpolated, there is a high possibility that the interpolation band itself is some harmonic components of the original spectrum. The signal with the expanded band is more approximate to the original signal before band limitation.
According to another aspect achieving the first object of the invention, the frequency interpolating device comprises: spectrum generating means for generating short period spectra of the input signal; spectrum pattern deriving means for deriving short period spectrum patterns having a correlation in adjacent frequency bands having a same band width; spectrum envelope deriving means for deriving spectrum envelope information in the band whose frequency components are not suppressed; means responsive to the spectrum pattern deriving means and the spectrum envelope driving means for synthesizing a frequency spectrum signal for interpolating the suppressed frequency band; and means for adding the synthesized spectrum signal to the input signal. In this device, the synthesized spectrum signal contains the frequency components in the suppressed frequency band, the derived spectrum pattern and the level determined by the spectrum envelope information. Typically, the input signal is a PCM signal obtained by sampling and quantizing an analog audio signal.
According to another aspect of the invention, there is provided a frequency interpolating method of receiving an input signal of an original signal whose frequency components in a particular frequency band was suppressed, approximately recovering suppressed frequency components and reproducing a signal approximate to the original signal, wherein: short period spectra are obtained from a frequency band having frequency components not suppressed; a short period spectrum of the frequency components in the suppressed frequency band is estimated in accordance with repetition of a spectrum pattern in the frequency band having frequency components not suppressed, and the estimated short period spectrum pattern is synthesized and added to the input signal.
In order to achieve the second object of the invention, in the frequency interpolating device of the invention and in a frequency interpolating system for receiving an input signal of an original signal whose frequency components in a particular frequency band was suppressed, approximately recovering suppressed frequency components and reproducing a signal approximate to the original signal, the device and system comprise: means for judging whether the particular frequency band of the original signal contains frequency components having a predetermined level or higher and generating identification data representative of a presence/absence of the frequency components having the predetermined level or higher; signal conversion means for suppressing the frequency components of the original signal in the particular frequency band and subjecting the original signal to a predetermined signal conversion process; means for superposing the identification data upon the converted signal and transmitting the identification data and the converted data; judging means for receiving a transmitted signal, checking the identification data contained in the signal, and judging a presence/absence of the frequency components in the particular frequency band; branch control means for controlling to output the received signal to an external if said judging means judges that the particular frequency band does not contain the frequency components and to input the received signal to succeeding signal processing means if the judging means judges that the particular frequency band contains the frequency components; and signal processing means responsive to the received signal from the control means for performing an inverse conversion process of the predetermined signal conversion process and an interpolation process of approximately synthesizing and adding the frequency components in the suppressed frequency band. More specifically, the predetermined signal conversion process is a data compression process and the inverse conversion process to be executed by said signal processing means is a data decompression process. The interpolation process to be executed by said signal processing means includes (i) a short period spectrum analysis process, (ii) a process of deriving short period spectrum patterns in adjacent frequency bands having a correlation, and (iii) a process of deriving spectrum envelope information.
With the frequency interpolating system, the identification data is generated which is representative of whether the spectrum of the original signal is distributed to the suppressed frequency band. If the identification data indicates an existence of a spectrum in the suppressed frequency band, a portion of the spectrum with a high correlation of the signal to be interpolated is added along an envelope to the high frequency side of the signal to be interpolated to thereby expand the band. The added spectrum can be regarded as some harmonic components of the original spectrum. Therefore, if the signal to be interpolated has a limited band, the signal with the expanded band is approximate to the original signal before the band limitation. If the identification data indicates no existence of the spectrum in the interpolation band, the signal to be interpolated is output without spectrum addition.
As a result, even if the received signal is a signal to be interpolated having the suppressed spectrum components in some bands of an original signal, or a signal representative of the original signal which does not contain the spectrum components in the bands, a signal approximate to the original signal can be recovered. If the signal is an audio signal, the audio signal of a high sound quality can be recovered.
The above-described frequency interpolating system has an integrated arrangement of a signal transmission side (including an encoder) and a signal receiving side (including a decoder). The invention may be embodied only by the reception side (decoder side). In this case, a frequency interpolating device for receiving an input signal of an original signal whose frequency components in a particular frequency band was suppressed, approximately recovering suppressed frequency components and reproducing a signal approximate to the original signal, comprises: means for receiving a first signal obtained by subjecting the original signal whose signal components in the particular frequency band were suppressed to a predetermined signal conversion process and a second signal superposed upon the first signal of identification data representative of whether the particular frequency band of the original signal contains the frequency components having a predetermined level or higher; judging means for checking the identification data contained in the received signal and judging a presence/absence of the frequency components in the particular frequency band; branch control means for controlling to output the received signal to each part if the judging means judges that the particular frequency band does not contain the frequency components and to input the received signal to succeeding signal processing means if the judging means judges that the particular frequency band contains the frequency components; and signal processing means responsive to the received signal from the branch control means for performing an inverse conversion process of the predetermined signal conversion process and an interpolation process of approximately synthesizing and adding the frequency components in the suppressed frequency band.
Similar to the frequency interpolating device, in order to achieve the first object of the invention, there is provided a frequency interpolating method which comprises: a step of judging whether the particular frequency band of the original signal contains frequency components having a predetermined level or higher and generating identification data representative of a presence/absence of the frequency components having the predetermined level or higher; a step of suppressing the frequency components of the original signal in the particular frequency band and subjecting the original signal to a predetermined signal conversion process; a step of superposing the identification data upon the converted signal and transmitting the identification data and the converted data; a judging step of receiving a transmitted signal, checking the identification data contained in the signal, and judging a presence/absence of the frequency components in the particular frequency band; a branch control step of controlling to output the received signal to an external if the judging step judges that the particular frequency band does not contain the frequency components and to input the received signal to a succeeding signal processing step only if the judging step judges that the particular frequency band contains the frequency components; and a signal processing step, responsive to the received signal from the branch control step, of performing an inverse conversion process of the predetermined signal conversion process and an interpolation process of approximately synthesizing and adding the frequency components in the suppressed frequency band.